CI: add required workflow rules

Required for pipelines to run after some infrastructure changes, see
freedesktop/freedesktop#438

.gitlab-ci.yml

@@ -11,6 +11,13 @@ include:
     file: '/templates/ci-fairy.yml'
 
 
+workflow:
+  rules:
+    - if: $CI_PIPELINE_SOURCE == "merge_request_event"
+    - if: $CI_COMMIT_BRANCH && $CI_OPEN_MERGE_REQUESTS
+      when: never
+    - if: $CI_COMMIT_BRANCH
+
 variables:
   FDO_UPSTREAM_REPO: 'libevdev/python-libevdev'
   ARCH_PKGS: 'flake8'